Working on 12VDC wiring

[Nrollins]

New to 12VDC electronics, so excuse what may be a naive question. I’m going to be doing some small 12V mods to my 1640 PUP, and wondering if I need to disconnect the + lead from my battery before doing work? I’ve read some things saying you can cut/touch/work on 12V wiring that are live and not risk electrocution, but that seems hard to understand (given all my experience is with 120VAC.) Thoughts?

[mac1_131]

That is generally true, but you can still damage/burn up things by sorting it to ground.

But yes you can usually touch it without harm, due to the lower voltage.

[Nrollins]

Originally Posted by mac1_131

That is generally true, but you can still damage/burn up things by sorting it to ground.

But yes you can usually touch it without harm, due to the lower voltage.

Overall though, to be safe, is there a mode of shutting off you recommend? Is there a shutoff at the fuses, or should I disconnect one or both leads at the battery?

[PhilFromMaine]

Originally Posted by Nrollins

Overall though, to be safe, is there a mode of shutting off you recommend? Is there a shutoff at the fuses, or should I disconnect one or both leads at the battery?

I generally disconnect the negative and tape with electrical tape or if I have a disconnect switch, I use that when I do electrical work. I have never had any arcing issues. Arching issues are the danger with 12v. Oh yes, I always wear safety glasses. It took a serious eye injury to knock some sense into me. LOL

[DSurette]

You could pull the fuse that is on the circuit you want to work on.

[PenJoe]

The proper way to disconnect a battery is to remove the negative cable first then the positive. To reconnect the battery, the positive cable first, then the negative second. The same goes when connecting/disconnecting cables to charge a battery. To answer the question of disconnecting the positive cable to safely work on 12 volt, disconnect both cables.

[Larry-NC]

Originally Posted by Nrollins

New to 12VDC electronics, so excuse what may be a naive question. I’m going to be doing some small 12V mods to my 1640 PUP, and wondering if I need to disconnect the + lead from my battery before doing work? I’ve read some things saying you can cut/touch/work on 12V wiring that are live and not risk electrocution, but that seems hard to understand (given all my experience is with 120VAC.) Thoughts?

Just to calm your fear, have you ever held the a 9v battery--the kind with two snap fasteners on the end, used in smoke detectors? Put your finger across the two snaps? Nothing happens, right? 12v is not much more.

Neither voltage is high enough to force current through dry skin. If you were working with an open cut, you might feel a tiny tingle.

The IEC (international safety commission) has established 42 volts as the threshold for dangerous voltage. Anything below that is safe and does not pose a threat from electric shock. Electrocution is NOT an issue.

The work you will be doing is likely all downstream of a fuse. A short circuit would blow the fuse before you even see the spark. Nothing to worry about there.

This is not to say that 12v is completely harmless. The current that's available at the battery terminals and cables is substantial. If you did something like connect the ends of a wire to the two battery terminals, it would glow red pretty quickly and could cause a burn. You would likely yank your hand away before damage occurred. This situation occurs because there is no fuse installed. If you are doing to do any work on non-fused wiring, be sure to disconnect the battery AND turn off the converter's circuit breaker or unplug from shore power.

[Sarahcat]

Originally Posted by PenJoe

The proper way to disconnect a battery is to remove the negative cable first then the positive. To reconnect the battery, the positive cable first, then the negative second. The same goes when connecting/disconnecting cables to charge a battery. To answer the question of disconnecting the positive cable to safely work on 12 volt, disconnect both cables.

Actually, it’s only necessary to disconnect one of the battery leads. Only one battery pole is fused. The only reason that the battery pole disconnection matters at all is in the case of a vehicle battery and in that case, it’s a matter of attaching the negative of a jumper cable to the vehicle frame, last. The reason for that caution is to attempt to move any arcing of the connection to be as far from the battery itself to mitigate the risk of igniting gasses from the battery electrolyte decomposition and having it go BOOM 🤯 Even that is not a likelihood.

[Larry-NC]

Originally Posted by Sarahcat

Actually, it’s only necessary to disconnect one of the battery leads. Only one battery pole is fused. The only reason that the battery pole disconnection matters at all is in the case of a vehicle battery and in that case, it’s a matter of attaching the negative of a jumper cable to the vehicle frame, last. The reason for that caution is to attempt to move any arcing of the connection to be as far from the battery itself to mitigate the risk of igniting gasses from the battery electrolyte decomposition and having it go BOOM 🤯 Even that is not a likelihood.

More specifically, think about this:

The negative battery cable is connected to the body and engine ground.

If you start to remove the positive cable first (or replace it last) and your wrench touches a chassis or engine part, you will get a nice big spark and arc. (I have a wrench or two with chunks out of them.)

If you remove the negative cable first (or replace it last), there is:
--No problem if the wrench on the negative terminal touches the chassis since they are already connected--no voltage difference, and
--No problem if the wrench on the positive terminal touches the chassis since the negative terminal is disconnected; there is no circuit, hence no spark or arc,

A dead or low battery gives off hydrogen--highly inflammable and explosive. All it takes is a little spark near the battery to ignite it. Ask my neighbor's husband who didn't know that and lost an eye checking water level in a dead battery by lighting a match and peering in.

[DouglasReid]

Originally Posted by Larry-NC

More specifically, think about this:

The negative battery cable is connected to the body and engine ground.

If you start to remove the positive cable first (or replace it last) and your wrench touches a chassis or engine part, you will get a nice big spark and arc. (I have a wrench or two with chunks out of them.)

If you remove the negative cable first (or replace it last), there is:
--No problem if the wrench on the negative terminal touches the chassis since they are already connected--no voltage difference, and
--No problem if the wrench on the positive terminal touches the chassis since the negative terminal is disconnected; there is no circuit, hence no spark or arc,

A dead or low battery gives off hydrogen--highly inflammable and explosive. All it takes is a little spark near the battery to ignite it. Ask my neighbor's husband who didn't know that and lost an eye checking water level in a dead battery by lighting a match and peering in.

Larry is right on the money.

Disconnect your Neg cable first before doing ANYTHING electrical on your vehicle or camper.

Then replace that Negative cable LAST!!

[eric1514]

On a motorhome, be aware that disconnecting the ground from the house battery(s) will not prevent you from shorting the positive terminal to the RV. The chassis battery is also grounded to the frame.

[Nrollins]

Originally Posted by DouglasReid

Larry is right on the money.



Disconnect your Neg cable first before doing ANYTHING electrical on your vehicle or camper.



Then replace that Negative cable LAST!!

Thanks all, so much help. This all makes sense. I will likely remove both disconnects (negative first) to be super safe, but one last question...

If I were to only remove the negative from terminal, do I have to be careful where I set it? If it touches the earth or the chassis, will I still have a live system? Where can I carefully place the negative lead? (I’ve only ever jumped cars, or fully removed leads to replace a battery.)

[jk510]

As long as you remove the cable at the battery you can put it anywhere. The other end is already connected to the frame/body.

Originally Posted by Nrollins

Thanks all, so much help. This all makes sense. I will likely remove both disconnects (negative first) to be super safe, but one last question...

If I were to only remove the negative from terminal, do I have to be careful where I set it? If it touches the earth or the chassis, will I still have a live system? Where can I carefully place the negative lead? (I’ve only ever jumped cars, or fully removed leads to replace a battery.)

[Nrollins]

Originally Posted by jk510

As long as you remove the cable at the battery you can put it anywhere. The other end is already connected to the frame/body.

Ok thanks, I had understood that the negative was the ground. Probably safer just to disconnect all

[PenJoe]

Originally Posted by Sarahcat

Actually, it’s only necessary to disconnect one of the battery leads. Only one battery pole is fused. The only reason that the battery pole disconnection matters at all is in the case of a vehicle battery and in that case, it’s a matter of attaching the negative of a jumper cable to the vehicle frame, last. The reason for that caution is to attempt to move any arcing of the connection to be as far from the battery itself to mitigate the risk of igniting gasses from the battery electrolyte decomposition and having it go BOOM 🤯 Even that is not a likelihood.

What was suggested in my post is the recommended and safest way to connect or disconnect a battery. The reason for the post was that the OP was asking to disconnect the positive battery cable. To prevent arching, the negative wire/cable should be the first to disconnect and the last to connect. I have done a fair share of wiring including converting a 6 volt system to 12 volts, vehicle wiring harnesses from scratch, trailer wiring, etc. And yes, the positive side has the fuse.